This invention relates to squelch systems. In particular this invention relates to squelch systems used in frequency modulation (FM) receives to disable stages of the receiver when signal is absent.
Typical squelch circuits in FM receivers test the output of the discriminator and disable the output of the audio amplifier stage of the receiver when there is no signal present on the channel. These squelch circuits therefore require that the discriminator remain active in the standby mode of the receiver. While squelch circuits principally eliminate unwanted white noise from being heard at the speaker output they can also substantially extend the life of a battery powering a portable FM receiver by powering down unneeded stages in the receiver when signal is absent from the channel.
Digital signal processors (DSP's), which use relatively large amounts of power and which are becoming commonplace in FM receivers, frequently perform the function of a discriminator, a function previously performed by relatively low-power analog circuits. Powering a DSP however to continuously perform a discriminator function to squelch the radio's audio system would waste substantial amounts of power. Using a DSP squelch algorithm not requiring the output of a discriminator but permitting the DSP to be disabled when its not required could substantially extend the life of a battery powering a portable radio that uses DSP's.